In radar altimeter operation, during aircraft roll or pitch maneuvers, it is possible for the altimeter track (range) gate to slide off the true altitude because the signal level is not maintained with sufficient accuracy. When the aircraft banks errors in the altitude can be generated due to coupled-control-loop induced positioning errors between the track gate and the level gate that are positioned relative to each other at a fixed separation. Likewise, variations in the terrain with respect to the attitude of the aircraft cause errors or inaccuracies due to the coupled-control-loop induced positioning errors. In the worst case, these positioning errors can result in coupled control loop oscillations that result in oscillations in the altitude value the radar altimeter reports. Such errors can cause unsafe flying conditions especially for aircraft that bank at large angles or fly over steep terrain, especially if these occur at low altitudes and the size of the altitude oscillations is a large fraction of the actual altitude.
When the airborne vehicle banks, the shape of the received waveform, often called the terrain echo, degrades. Ideally, the terrain echo would resemble a square pulse. Because the transmitted signal from the radar altimeter spreads across the ground, the shape of the terrain echo more closely resembles a triangular pulse with a steep slope on the leading edge and a shallower slope on the falling edge. When the received terrain echo signal spreads out in this manner, the track gate slides outbound away from the peak. The track gate control loop is designed to respond faster than the level gate. Thus, with a fixed separation between the track and level gates, the level gate is forced outbound along with the track gate. The track gate and level gate continue to slide outbound away from the peak until the amplitude level of the signal drops enough for the amplitude of the signal within the track gate to be at the track reference level. Once the track gate amplitude is at the track reference level the track gate control loop is satisfied.
At this point, the amplitude of the signal within the level gate is too high. This forces the level control loop to decrease the overall amplitude of the terrain echo. However, as level control loop pulls down the peak signal amplitude, the amplitude of the signal in the track gate falls below the track reference level. This causes the track control loop to slide the track gate position outbound until the signal within the track gate is at the track reference level. If the terrain echo signal has a sufficiently broad peak, the level gate will eventually measure a relatively constant signal level over a range of positions and the control loops for both the track gate and level gate will be satisfied. As defined herein, a gate slides inbound when it moves downward in altitude. Likewise, a gate slides outbound when it moves upward in altitude.
If the terrain echo peak is narrow, the level gate can be driven past the terrain echo peak. This results in a drop in level amplitude, which causes the level control loop to increase the signal level. This also causes the signal at the track point to rise and this rise causes the signal level in the track gate to rise above the track reference level. The track control loop will drive the track gate inbound until the signal level in the track gate is at the track reference level. Since the track gate and level gate are coupled together at a fixed separation from one another, the level gate is also pulled inbound and is pulled toward the terrain echo peak. As the level gate is pulled toward the peak, the level signal increases and the level control loop drives the signal level down. This also drives the level of the signal in the track gate down and this causes the track gate control loop to drive the track gate position outbound and the cycle repeats.
Coupled control loop oscillations and their impact on system stability are well known within the control system community. The impact on radar altimeters can be inferred from Merril Skolnik's reference book, “Radar Handbook.” in Section 18.8 of Skolnik's book, there is an extensive discussion of the impact of various forms of amplitude noise on pointing errors in tracking radar. Although Skolnik is primarily concerned with tracking and scanning radar systems, one can easily relate noise induced pointing angle errors in tracking radars to altitude errors in radar altimeters. In tracking radars, pointing angle is a critical system output. In radar altimeters, altitude output is the critical system output. Amplitude fluctuations in tracking radars induce pointing errors as the radar interprets target echo amplitude changes as changes in apparent target position. These same amplitude fluctuations are interpreted by radar altimeters as changes in apparent altitude. Thus, the noise introduced into the terrain echo signals by instability in the echo amplitude caused by coupled control loop oscillations will be interpreted as an apparent change in altitude. The gain control loop is relatively slow compared to the track loop and the terrain echo amplitude oscillations will be translated into altitude variations. These oscillations occur at a slow enough rate that they cannot be effectively removed by filtering or averaging without introducing an unacceptable lag in the response of the radar altimeter to actual changes in altitude.